The present invention is a dual seal intended to seal the space between a rotating shaft and a housing against the loss of fluid under high pressure within the housing. Such seals are typically used in pumps and compressors and are of the radial end face type.
Dual seals have previously been used in such environments. These prior art seals have taken different forms and operate in different manners. The prior art seals are illustrated in FIGS. 1-4 of the attached drawings. The pressure curve below each of these seals may be used as an aid to understanding their operation. Dashed arrows indicate the direction of leakage flow while a solid arrow represents the direction of intentional pumping across the seal faces.
A common dual seal is the "double seal" arrangement of FIG. 1. Such a seal comprises two radial end face seals oriented back to back along a drive shaft (DS) within a housing (H). Each face seal has a stationary ring (S) and a rotating ring (R) with their radial faces (F) biased into engagement with A bias (B). These seals are mounted within a chamber C in the housing which contains a buffer fluid pressurized from an external source (not shown). The arrangement is referred to as a back to back orientation because the back of one rotating ring (R) is opposed to the back of the other rotating ring. As depicted by pressure diagram, the fluid pressure to be sealed is to the left of the seal and the atmosphere is to the right. During normal operation, a lubricant is circulated within the chamber of this tandem seal at a pressure that is generally 5-20 PSI above the sealed pressure. The chamber pressure is necessary to aid the bias (B) in keeping the inboard seal closed. This arrangement is most desirable when corrosive liquids are being sealed because the metal parts of the seal are isolated within a non-corrosive buffer liquid. In this arrangement the outboard seal must accommodate more than system pressure.
Another type of a dual seal is the "tandem seal" of FIG. 2. Here, the seals are mounted in the same direction. A restricted interconnection between the sealed chamber and the buffer chamber (not shown) may be used to permit sealed fluid to enter the buffer chamber at pressures up to one half the sealed pressure. Thus, each component seal may accept and seal one half of the total pressure, as shown by the pressure chart. The tandem seal has the benefit of reducing the pressure which each seal must hold, but contrary to the double seal, the metal parts of a tandem seal are exposed to the sealed liquid.
A third type of dual seal is depicted in FIG. 3 which illustrates the disclosure of U.S. Pat. No. 3,894,741. The patent states that it is directed to a special application--submersible pumps. The outside of the seal housing is immersed in water. The outboard seal is mounted in the buffer chamber C which is filled with oil. The inboard seal is also mounted in oil and has spiral pumping grooves which pump oil into the buffer chamber and past the outboard seal to oppose water leakage into the seal pump housing.
Another type of dual seal is that sold by Borg-Warner Corporation under the designation "Type GU Mechanical Seal" (see attached advertisement). In this dual seal, the inboard seal is a liquid seal while the outboard seal is a gas seal. The inboard seal precludes or limits the leakage of liquid. Liquid which does leak past the inboard seal is drained from the chamber C by taps (not shown). The outboard seal acts as a gas seal and a backup seal. In the event of failure of the inboard liquid seal, the outboard gas seal may, for a limited time, act as a liquid seal but it is not designed to run in a liquid for extended periods of time.
The above illustrations represent the common types of dual seal applications as understood by this inventor. Reference should be made to U.S. Pat. No. 3,894,741, for a more comprehensive disclosure.
Each of these common types of dual seals have leakage or pumping action in a direction outwardly of the housing. They do not have a true, second, safety seal which operates in a fluid medium without load or wear until the first primary seal fails.